ARM: dts: at91: at91sam9g25ek: fix isi endpoint node

Add a reg property in the endpoint node as documented in
Documentation/devicetree/bindings/media/video-interfaces.txt

Solves:
Warning (unit_address_vs_reg): Node /ahb/apb/isi@f8048000/port/endpoint@0 has a unit name, but no reg property

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>